Athletes with mono are typically held out of all physical activity for a period of 30-50 days after diagnosis.
Mono is a disease called Mononucleosis. It is also known as "kissing disease".
It is an infectious disease in which one's spleen rupture or enlarged, it also causes fever, sore throat etc. That's why athletes are held out of all types of physical activities for some time period. Athletes can resume their activities when all the symptoms are gone.